1. Field of the Invention
This invention relates to (InGa)As-lnP heterojunction devices utilizing a InP:.o slashed. collector to extract electrons from a (InGa)As:p layer and more particularly to heterojunction devices having a smooth and monotonic potential energy gradient at the heterojunction for establishing drift of the conduction electrons from the (InGa)As:p layer to the InP:.o slashed. layer.
2. Description of the Prior Art
Heretofore, the efficient transfer of electrons from (InGa)As:p to InP:n has been achieved in the collector junction of heterojunction bipolar transistors through the use of grading layers to smooth the conduction band discontinuity. These devices rely on the built-in field of the extrinsic heavily-doped (InGa)As:p base and InP:n collector or on externally-applied reverse biases of the heavily doped base-collector junction to provide the electric field necessary to remove the conduction-band barriers at the junction. While these devices provide a smooth and monotonic variation in the conduction band at the heterojunction, they are limited in use to extrinsic p-n heterojunctions which can support a sufficiently large electric field. The incorporation of a intrinsic layer into this junction, as may be desired for instance to reduce the base-collector capacitance, would decrease the electric field at the junction and cause the formation of potential energy barriers to the conduction electrons.
The application of any such grading structure to (InGa)As photodetectors has not been reported. Conventional p-i-n (InGa)As-InP photodetectors utilize a low-doped (InGa)As absorbing layer and extrinsic n- and p-type InP contact layers to circumvent problems associated with the conduction and valence band discontinuities. This approach precludes the use of thick InP:.o slashed. layers which would trap photogenerated electrons at the band discontinuity and greatly reduce the efficiency of the photodiode. The insertion of a thick intrinsic region is necessary to reduce the junction capacitance and improve the frequency response of the photodiode. Conventional photodiodes rely on the depletion of the (InGa)As absorber region to reduce the junction capacitance. This approach requires the use of large applied voltages to deplete the absorber layer due to its residual doping background and results in large dark currents due to thermally generated electrons in the small bandgap depletion region.